Where can mixing (and thus, intermodulation) of RF signal occur in a typical wireless microphonesystem? There are multiple possible answers to this question.
Shot in the dark after some research that I'm not sure is applicable:
The two parameters that most affect the interfering signal level into the transmitter's output circuit are the output loading and the circuit's frequency selectivity.
I would think the receiving antenna - if active, the combiner - if active, and the receivers themselves. I suppose it's possible that it could occur in the passive receiving components, but I'm not that wireless savvy. It might even occur in the transmitter if it's hit with enough of the right signal?
Particularly in IEM systems where you get a number of transmitters stacked on top of each other, in many cases literally, you are most definitely getting Intermods in the transmitters...